Process for enhancing dry compressive strengsth in iron ore pelletizing

ABSTRACT

A process for producing green iron ore pellets comprises the inclusion of Direct Reduced Iron or a metallic iron-containing material into the pellet mixture to enhance Direct Compressive Strength properties of the green pellets.

FIELD OF THE INVENTION

The present invention relates to the production of green iron ore pellets with enhanced Dry Compressive Strength (DCS).

In particular, the present invention has reference to an improved process for increasing DCS of green pellets in a pelletizing system. DCS is defined as the mechanical loading measured in kilograms required to fracture a dry ball.

BACKGROUND OF THE INVENTION

Green pellets are usually produced in spherical form from agglomerates, nominally 8-18 mm diameter, and are generally so formed by rolling finely ground iron ore, additives such as limestone, dolomite, anthracite etc together with a binder such as bentonite, hydrated lime or organic binders in a damp state.

It is well known that pellets must possess sufficient DCS to withstand pressure exerted in a pellet bed during the drying phase of the pelletization process in which the pellets travel as a bed on a traveling grate, in order to maintain bed permeability. This requirement assumes especial importance in the ‘Straight-Grate’ pelletizing process in which the bed depth is 2.5 times that of the ‘Grated-Kiln’ process.

Currently, there is a tendency in pelletizing to lower the use of bentonite by using caustic soda (NaOH) concomitantly or to use carboxymethyl cellulose (CMC) based organic binders in lieu of bentonite in order to reduce the acid gangue (SiO₂+Al₂O₃) content especially in Direct Reduction (DR) grade pellets. It has been observed in both cases that DCS is reduced substantially.

Hitherto, no solutions have been found to prevent the decrease in DCS upon using lower dosages of bentonite or using organic binders in lieu of bentonite in pelletizing.

Accordingly, there is a need for an improved process for the production of green iron ore pellets having enhanced Dry Compressive Strength.

SUMMARY OF THE INVENTION

It is therefore a general object of the present invention to provide an improved process for the production of green iron ore pellets having enhanced Dry Compressive Strength (DCS).

An advantage of the present invention is that the process for the production of green iron ore pellets increases the DCS of such green pellets whilst on a traveling grate machine after removing the free moisture content.

Another advantage of the present invention is that the process for the production of green iron ore pellets enhances DCS levels with an admixture of constituents including bentonite at lower dosage levels than hitherto.

A further advantage of the present invention is that the process for the production of green iron ore pellets allows the reduction of acid gangue content in the pellets by virtue of being able to employ lower dosages of bentonite or organic binders in lieu of bentonite.

A still further advantage of the present invention is that the process for the production of green iron ore pellets enhances DCS levels by admixture of Direct Reduced Iron or other material containing same with binders before blending and mixing with iron ores and additives.

In accordance with a first aspect of the present invention, there is provided a process for increasing the Dry Compressive Strength of green iron ore pellets including the steps of introducing Direct Reduced Iron fines into a pellet mix of inter alia iron ore and binder, forming spherical green pellets from the resultant pellet feed, and introducing said green pellets into a drying stage to remove moisture before the heat-hardening or indurating process to produce product pellets as feedstock for Direct Reduction (DR) or Blast Furnace (BF) processes.

In an alternative embodiment of the present invention there is provided a process for increasing the Dry Compressive Strength of green iron ore pellets including the steps of introducing Direct Reduced Iron (DRI) fines or other form of metallic iron-containing material in a solid-state into a pellet mix of inter alia iron ore and binder, forming spherical green pellets from the resultant pellet feed, and introducing said green pellets into a drying stage to remove moisture from the pellets to generate dry pellets before the indurating process.

In a further embodiment of the present invention there is provided a process for increasing the Dry Compressive strength of green iron ore pellets including the steps of mixing Direct Reduced Iron in pulverized form with a binder, blending the resultant mixture with a mixture of inter alia iron ores to produce a pellet feed, forming spherical green pellets from the feed, and introducing said green pellets into a drying stage to remove moisture from the pellets to generate dry pellets before the indurating process.

The process includes a step of mixing all the ingredients of pellets such as iron ore, additives and the binder adjusting the moisture content of the mix at a level suitable for pelletizing.

The binder may be bentonite included in the mixture in dosages of lower levels than conventionally employed.

The binder may be an organic binder.

The organic binder may be of the type sold under the Trade Mark Peridur® which is a sodium carboxymethyl cellulose-containing binder produced by Akzo Chemicals of the Netherlands.

Caustic soda may be added into the pellet mixture.

Other additives may include, for example and not by way of limitation, any one or more of the following, namely limestone, dolomite, serpentine, anthracite, petroleum coke, boron compounds commonly employed in the pelletizing industry.

Other additives may be included in the pellet feed.

The Direct Reduced Iron (DRI) fines inclusion may lie in the range of 2% to 5% by weight.

The metallic iron-containing material inclusion may lie in the range of 2% to 5% by weight.

The step of introducing DRI or other metallic iron-containing material fines may be by admixture with the other constituents, such as iron ore, binder and additives, to be ground together prior to pelletization. In the alternative, the DRI fines or other metallic iron-containing material fines together with the binder may be pre-mixed prior to admixture with the other constituents, which with lower dosages of for example bentonite ensures good mixing prior to pelletization.

The drying stage may be carried out in conventional machines used for such a purpose.

Other objects and advantages of the present invention will become apparent from a careful reading of the detailed description provided herein, with appropriate reference to the accompanying examples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

Spherical green pellets were prepared in a 60 cm diameter laboratory balling disc using 0.5% bentonite for a magnetite ore and two types of hematite ores. DRI or metallic iron-containing material was added in the amounts of 2% and 5% for each test with a reference mixture containing no DRI. The binder used was bentonite. The procedures for pelletizing the feed are well known in the art and are thus not described herein.

The green spheres so produced were then dried at 105° C. for one hour. DCS was subsequently determined using the procedure set out in ISO 4700:1996.

The test results and the analyses of the mixtures are shown in Tables 1A, 1B and 1C for Test 1A, Test 1B and Test 1C, respectively.

TABLE 1A Test 1A (Ore 1) with bentonite TEST 1 TEST 2 TEST 3 Blend ratio (%) Blend ratio (%) Blend ratio (%) Ore 1 99.5 97.5 94.5 Bentonite 0.5 0.5 0.5 DCL-DRF — 2.0 5.0 Moisture (%) 7.31 7.22 7.8 BLAINE¹ (cm2/g) Ore 1: 1959, DCL: 1905, Bentonite: 4686 GREEN BALL SIZE: 9 to 11 mm Average of 12 observations DROP NOS Avg 3.2 3.2 3.1 Max 4 4 4 Min 2 2 2 GCS² (Kg/p) Avg 1.79 1.42 2 Max 2.1 1.75 2.5 Min 1.5 1.08 1.3 DCS³ (Kg/p) Avg 3.96 4.79 6.77 Max 4.88 5.6 7.78 Min 2.83 3.67 5.5 Chemical Analysis T.Fe⁴ 66.7 67.06 67.16 M.Fe⁵ 0.0 0.37 1.62 FeO 22.5 23.14 25.89 C 0.145 0.32 0.195 ¹Specific surface area of iron ore powder before pelletization, measured by air permeability methods with a special apparatus called the “Blaine” apparatus, in square centimeters per gram; ²Green Compressive Strength (of green pellets), in kilogram per pellet; ³Dry Compressive Strength, measured in kilogram per pellet; ⁴Total iron (Fe) content in wt %, including in ferric, ferrous and metallic states; ⁵Metallic Fe content, in wt %.

TABLE 1B Test 1B (Ore 2) with bentonite TEST 1 TEST 2 TEST 3 Blend ratio (%) Blend ratio (%) Blend ratio (%) Ore 2 99.5 97.5 94.5 Bentonite 0.5 0.5 0.5 DCL-DRF — 2.0 5.0 Moisture (%) 6.99 6.9 6.92 BLAINE (cm2/g) Ore 2: 1930, DCL: 1905, Bentonite: 4686 GREEN BALL SIZE: 9 to 11 mm Average of 12 observations DROP NOS Avg 2.8 3.7 2.8 Max 3 5 3 Min 2 3 2 GCS (Kg/p) Avg 1.29 1.28 1.17 Max 1.54 1.59 1.39 Min 1.14 0.89 0.94 DCS (Kg/p) Avg 5.34 5.8 6.94 Max 8.94 6.72 7.94 Min 3.58 4.48 5.58 Chemical Analysis T.Fe 66.86 68.12 68.64 M.Fe 0.0 1.01 1.97 FeO 0.14 3.28 6.98 C 0.05 0.09 0.109

TABLE 1C Test 1C (Ore 3) with bentonite TEST 1 TEST 2 TEST 3 Blend ratio (%) Blend ratio (%) Blend ratio (%) Ore 3 99.5 97.5 94.5 Bentonite 0.5 0.5 0.5 DCL-DRF — 2.0 5.0 Moisture (%) 7.1 7.24 7.22 BLAINE (cm2/g) Ore 3: 2129, DCL: 1905, Bentonite: 4686 GREEN BALL SIZE: 9 to 11 mm Average of 12 observations DROP NOS Avg 3.3 3.2 3.3 Max 4 4 4.1 Min 2 3 3 GCS (Kg/p) Avg 1.22 1.19 1.12 Max 1.38 1.45 1.35 Min 0.88 0.83 0.9 DCS (Kg/p) Avg 3.39 4.16 4.53 Max 4.26 4.53 5.99 Min 2.73 3.32 2.92 Chemical Analysis T.Fe 65.97 67.72 67.83 M.Fe 0.0 1.45 2.38 FeO 0.82 4.56 3 C 0.035 0.069 0.085

It will readily be seen that the DCS of the green pellets with DRI or equivalent additive is enhanced in comparison to that of green pellets with no additive of this type.

EXAMPLE 2

The same procedure as set forth in Example 1 with the same ores was adopted except that instead of bentonite, 0.4 wt % organic binder called Peridur® and 0.02 wt % caustic soda were used.

The test results and the analyses are shown infra in Tables 2A, 2B and 2C for Test 2A, Test 2B and Test 2C, respectively.

TABLE 2A Test 2A (Ore 1) with organic binder TEST 1 TEST 2 TEST 3 Blend ratio (%) Blend ratio (%) Blend ratio (%) Ore 1 99.94 97.94 94.94 Peridur ® 0.04 0.04 0.04 Caustic soda 0.02 0.02 0.02 DCL-DRF — 2.0 5.0 Bentonite — — — Moisture (%) 6.35 6.49 6.75 BLAINE (cm2/g) Ore 1: 1934, DCL: 1907 GREEN BALL SIZE: 9 to 11 mm Average of 12 observations DROP NOS Avg 3 4.6 3.7 Max 3 5 4 Min 3 4 3 GCS (Kg/p) Avg 1.19 1.55 1.23 Max 1.69 2.28 1.49 Min 0.51 1.26 0.97 DCS (Kg/p) Avg 1.9 4.53 4.89 Max 2.45 6.22 6.14 Min 1.41 3.57 3.56 Chemical Analysis T.Fe 67.21 67.98 68.75 M.Fe 0.24 1.16 2.17 FeO 18.79 20.83 25.87 C 0.151 0.169 0.179

TABLE 2B Test 2B (Ore 2) with organic binder TEST 1 TEST 2 TEST 3 Blend ratio (%) Blend ratio (%) Blend ratio (%) Ore 2 99.94 97.94 94.94 Peridur ® 0.04 0.04 0.04 Caustic soda 0.02 0.02 0.02 DCL-DRF — 2.0 5.0 Bentonite — — — Moisture (%) 6.41 7 6.95 BLAINE (cm2/g) Ore 2: 1908, DCL: 1907 GREEN BALL SIZE: 9 to 11 mm Average of 12 observations DROP NOS Avg 3 3.3 2.7 Max 3 4 3 Min 3 3 2 GCS (Kg/p) Avg 1.09 1.72 1.46 Max 1.49 1.92 1.74 Min 0.97 1.41 1.21 DCS (Kg/p) Avg 3.17 5.89 5.45 Max 4.22 7.26 6.22 Min 2.66 4.14 4.57 Chemical Analysis T.Fe 68.25 68.94 68.59 M.Fe 0.16 1.37 1.99 FeO 0.34 5.65 2.43 C 0.036 0.047 0.649

TABLE 2C Test 2C (Ore 3) with organic binder TEST 1 TEST 2 TEST 3 Blend ratio (%) Blend ratio (%) Blend ratio (%) Ore 3 99.94 97.94 94.94 Peridur ® 0.04 0.04 0.04 Caustic soda 0.02 0.02 0.02 DCL-DRF — 2.0 5.0 Bentonite — — — Moisture (%) 6.7 6.98 6.84 BLAINE (cm2/g) Ore 3: 1946, DCL: 1907 GREEN BALL SIZE: 9 to 11 mm Average of 12 observations DROP NOS Avg 3.3 3.2 3.2 Max 4 4 4 Min 3 3 3 GCS (Kg/p) Avg 1.73 1.66 1.44 Max 2.1 2.26 1.72 Min 1.31 1.23 1.13 DCS (Kg/p) Avg 2.43 3.04 3.00 Max 3 3.55 3.62 Min 2.09 2.75 2.31 Chemical Analysis T.Fe 66.29 66.43 66.42 M.Fe 0.17 1.75 2.27 FeO 0.56 0.92 2.9 C 0.063 0.049 0.078

As can again be seen DCS is enhanced.

EXAMPLE 3

Similar tests were conducted as those described with reference to Examples 1 and 2 except that mill scale was substituted for DRI fines and organic binder was used.

The results are shown in Tables 3A, 3B and 3C for Test 3A, Test 3B and Test 3C, respectively.

TABLE 3A Test 3A (Ore 1) with mill scale and organic binder TEST 1 TEST 2 TEST 3 Blend ratio (%) Blend ratio (%) Blend ratio (%) Ore 1 99.94 97.94 94.94 Peridur ® 0.04 0.04 0.04 Caustic soda 0.02 0.02 0.02 Mill Scale — 2 5 Bentonite — — — Moisture (%) 6.35 6.65 6.66 BLAINE (cm2/g) Ore 1: 1934, Mill Scale: 1941 GREEN BALL SIZE: 9 to 11 mm Average of 12 observations DROP NOS Avg 3 3.3 3.2 Max 3 4 4 Min 3 3 3 GCS (Kg/p) Avg 1.19 0.97 1.2 Max 1.69 1.06 1.32 Min 0.51 0.84 0.94 DCS (Kg/p) Avg 1.9 1.59 1.75 Max 2.45 1.82 2.08 Min 1.41 1.44 1.07 Chemical Analysis T.Fe 67.21 67.99 67.95 M.Fe 0.24 0.46 0.29 FeO 18.79 25.55 25.14 C 0.151 0.141 0.173

TABLE 3B Test 3B (Ore 2) with mill scale and organic binder TEST 1 TEST 2 TEST 3 Blend ratio (%) Blend ratio (%) Blend ratio (%) Ore 2 99.94 97.94 94.94 Peridur ® 0.04 0.04 0.04 Caustic soda 0.02 0.02 0.02 Mill Scale — 2 5 Bentonite — — — Moisture (%) 6.41 6.53 6.74 BLAINE (cm2/g) Ore 2: 1908, Mill Scale: 1941 GREEN BALL SIZE: 9 to 11 mm Average of 12 observations DROP NOS Avg 3 3.1 3 Max 3 4 3 Min 3 3 3 GCS (Kg/p) Avg 1.09 1.01 0.95 Max 1.49 1.22 1.16 Min 0.97 0.89 0.71 DCS (Kg/p) Avg 3.17 2.88 2.18 Max 4.22 3.6 2.45 Min 2.66 2.2 1.02 Chemical Analysis T.Fe 68.25 68.44 68.25 M.Fe 0.16 0.15 0.69 FeO 0.34 7.19 5.86 C 0.036 0.061 0.096

TABLE 3C Test 3CA (Ore 3) with mill scale and organic binder TEST 1 TEST 2 TEST 3 Blend ratio (%) Blend ratio (%) Blend ratio (%) Ore 3 99.94 97.94 94.94 Peridur ® 0.04 0.04 0.04 Caustic soda 0.02 0.02 0.02 Mill Scale — 2 5 Bentonite — — — Moisture (%) 6.7 6.75 6.76 BLAINE (cm2/g) Ore 3: 1946, Mill Scale: 1941 GREEN BALL SIZE: 9 to 11 mm Average of 12 observations DROP NOS Avg 3.3 3 3 Max 4 3 3 Min 3 3 3 GCS (Kg/p) Avg 1.73 0.98 0.87 Max 2.1 1.14 1.01 Min 1.31 0.84 0.66 DCS (Kg/p) Avg 2.43 1.82 1.5 Max 3 2.63 1.83 Min 2.09 1.02 1.13 Chemical Analysis T.Fe 66.29 66.22 66.3 M.Fe 0.17 0.19 0.24 FeO 0.56 2.47 4.08 C 0.063 0.041 0.0492

Mill scale includes wustite and magnetite and these constituents do not have an enhancing effect upon DCS.

Table 4 below sets out the chemical analysis of the materials used in the Examples.

TABLE 4 Chemical analyses of materials used Chemical Analyses of Materials used % Ore 1 Ore 2 Ore 3 Mill Scale DRI Fines Bentonite Fe_(T) ¹ 67.15 68.44 66.17 71.76 84.60 10.95 FeO 24.60 0.22 0.71 56.05 15.50 nd Fe_(M) ² 0.00 0.00 0.00 0.12 56.50 0.00 SiO2 2.15 1.03 4.75 0.46 1.65 48.63 Al2O3 0.38 0.28 0.25 2.17 0.96 16.57 CaO 0.60 0.06 0.03 0.43 0.03 0.83 MgO 2.10 0.04 0.05 0.22 0.25 2.46 MnO 0.03 0.11 0.02 0.73 0.14 0.14 C nd nd nd 0.24 0.9 nd P 0.040 0.020 0.010 0.010 0.050 0.120 S 0.390 0.004 0.003 0.010 0.100 0.100 LOI³ −1.19 0.45 0.13 −7.28 −21.5 −7.28 ¹Total Fe-content; ²Metallic Fe-content; ³Loss-on-ignition; loss of weight of iron ore sample upon heating at 1000° C. for 30 minutes,

The present invention is of particular import for example when using Direct Reduction and Electric Arc Furnace production routes as the pellets so produced have a high iron content with low acid gangue content.

Although the present invention has been described with a certain degree of particularity, it is to be understood that the disclosure has been made by way of example only and that the present invention is not limited to the features of the embodiments described and illustrated herein, but includes all variations and modifications within the scope and spirit of the invention as hereinabove described. 

1. A process for increasing the Dry Compressive Strength of green iron ore pellets including the steps of introducing Direct Reduced Iron fines into a pellet mix of inter alia iron ore and binder, forming spherical green pellets from the resultant pellet feed, and introducing said green pellets into a drying stage to remove moisture from the pellets to generate dry pellets before the indurating process step of pelletizing.
 2. A process according to claim 1 wherein the Direct Reduced Iron inclusion lies in the range 2% to 5%.
 3. A process for increasing the Dry Compressive Strength of green iron ore pellets including the steps of introducing metallic iron-containing material in a solid-state into a pellet mix of inter alia iron ore and binder, forming spherical green pellets from the resultant pellet feed, and introducing said green pellets into a drying stage to remove moisture from the pellets to generate dry pellets before the indurating process.
 4. A process according to claim 3 wherein the metallic iron-containing material inclusion lies in the range 2% to 5%.
 5. A process for increasing the Dry Compressive strength of green iron ore pellets including the steps of mixing Direct Reduced Iron in pulverized form with a binder, blending the resultant mixture with a mixture of inter alia iron ores to produce a pellet feed, forming spherical green pellets from the feed, and introducing said green pellets into a drying stage to remove moisture from the pellets to generate dry pellets before the indurating process.
 6. A process according to claim 1 further including the step of adjusting the moisture content of the mix at a level suitable for pelletizing by mixing pellets ingredients selected from the group comprising iron ore, additives and the binder.
 7. A process according to claim 1 wherein the binder is bentonite.
 8. A process according to claim 1 wherein the binder is an organic binder.
 9. A process according to claim 8 wherein the organic binder is a sodium carboxymethyl cellulose-containing binder.
 10. A process according to claim 8 wherein caustic soda is added to the pellet mixture.
 11. A process according to claim 1 wherein additives selected from the group comprising limestone, dolomite, serpentine, anthracite, petroleum coke, and boron compounds are included in the pellet mixture. 